Golf balls are classified roughly into a thread wound golf ball and a solid golf ball. The thread wound golf ball is superior in hit feeling (feeling at the time of hitting) and control properties. The solid golf ball has a large impact resilience and attains a large flying distance but has a problem that hit feeling and control properties are inferior.
The thread wound golf ball is obtained by winding a thread rubber at a stretched state around a center to form a thread wound core, and then covering a cover on the thread wound core. The center of the thread wound golf ball is classified into two types, i.e. liquid center wherein a rubber bag referred to as a center bag is filled with a solution or pasty liquid, and solid center of a rubber crosslinked material having a high impact resilience.
Among them, the solid center is easily produced and is comparatively cheap and, therefore, it has been used as the center of the thread wound golf ball for middle- to high-class golfers, mainly.
To the contrary, the thread wound golf ball using the liquid center is superior to a conventional solid golf ball as well as thread wound golf ball using the above solid center in hit feeling and control properties. Therefore, highest-class golfers such as professional and top-amateur golfers like it. However, it is difficult to produce the thread wound golf ball using the liquid center and, therefore, the liquid center is applied only for a highest-grade thread wound golf ball at present.
Hereinafter, a process for producing a liquid center and a thread wound golf ball using it will be explained.
Firstly, a liquid center is made either by pouring a liquid into a mold cooled with a refrigeration medium, freezing the liquid to make a spherical core, covering a non-crosslinked rubber bag on this spherical core, or by pouring a liquid into a crosslinked rubber bag using a syringe and sealing a mark formed by an injection.
Then, the liquid center thus obtained is changed into a state that the liquid center is hard and has a shape retention by freezing, and a thread rubber is wound at a stretched state around the liquid center to form a thread wound core. The thread wound core is covered with a cover and, if necessary, the surface is painted to produce a thread wound golf ball.
The thread wound golf ball produced by using this liquid center has such characteristics that hit feeling is good and control properties are excellent. However, the impact resilience of the liquid center is lower than that of the solid center and, therefore, the flying distance is small. Accordingly, average golfers, who think flying distance important, do not like the golf ball using the liquid center. Therefore, it is necessary to use a thread rubber having a high impact resilience so as to obtain a thread wound golf ball which has a high impact resilience and attains a large flying distance, which results in rise in cost.
As described above, the thread wound golf ball using the liquid center has a such characteristics that the hit feeling is good and control properties are excellent. However, the flying distance is inferior to the other golf ball and, therefore, average golfers, who think the flying distance important, do not like the golf ball using the liquid center.